In a number of high energy ( or = 1 TeV/amu) nucleus-nucleus collisions observed in Japanese-American Cooperative Emulsion Experiment (JACEE) emulsion chambers, nonrandom spatial association of produced charged particles, mostly hadronic pairs, are observed. Similar narrow pairs are observed in about 100 events at much low energy (20 to 60 GeV/amu). Analysis shows that 30 to 50% of Pair abundances are understood by the Hambury-Brown-Twiss effect, and the remainder seems to require other explanations

Burnett, T. H.

Dake, S.

Fuki, M.

Gregory, J. C.

Hayashi, T.

Holynski, R.

Iwai, J.

Jones, W. V.

Jurak, A.

English

NASA Technical Reports Server

168 HE 1.4-5
OBSERVATIONOF DIRECT HADRONICPAIRS IN NUCLEUS-NUCLEUS
COLLISIONS IN JACEE EMULSIONCHAMBERS
The JACEE Collaboration +
T.H. Burnett(h), S. Dake(b), M. Fuki(c), J.C. Gregory(g), T. Hayashi(g),
T. Hayashi(b), R. Holynski(i), J. lwai((h), W.V. Jones(e), A. Jurak(i),
J.J. Lord(h), O. Miyamura(d), H. Oda(b), T. Ogata(a), T.A. Parnell(f),
T. Saito(a), T. Tabuki(a), Y. Takahashi(f), T. Tominaga(d),
B. Wilczynska(i), R.J. Wilkes(h), W. Wolter(i), and B. Wosiek(i)
+ Japanese-American-Cooperative-Emulsion-Experiment
(a) Institue for Cosmic Ray Research, University of Tokyo, Tokyo, Japan
(b) Department of Physics, Kobe University, Kobe, Japan
(c) Department of Physics, Okayama University of Science, Okayama, Japan
(d) Department of Applied Mathematics, Osaka University, Osaka, Japan
(e) Department of Physics & Astronomy, Louisiana State University, USA
(f) ES62, Space Science Laboratory, NASAMarshall Space Flight Center,
Huntsville, Alabama 35812, USA
(g) Department of Chemistry & Res. Inst. Univ. of Alabama, Huntsville, USA
(h) Department of Physics, University of Washington, Seattle, USA
(i) Institute of Nuclear Physics, Krakow, Poland
ABSTRACT
In a number of high energy (_ i TeV/amu) nucleus,nucleus
collisions observed in JACEE emulsion chambers, non-random
spatial association of produced charged particles, mostly
hadronic pairs, are observed. Similar narrow pairs are
observed in about i00 events at much low energy (20 - 60
GeV/amu) Analysis shows that 30 - 50 % of Pair abundances
are understood by the Hambury.Brown-Twiss effect, and the
remainder seems to require other explanations.
i. Introduction. Frequent association of produced particles in pairs
have been noticed in measurements of charged particle angular distributions
in nucleus-nucleus collisions. Several high energy events manifested
significant abundances in JACEE emulsion chambers. Statistical analy-
ses of larger samples(106 events) at lower energy (20 - 60 GeV/amu) also
indicate non-trivial pair abundances.
From the viewpoint of independent superposition models it is not un-
natural to expect weaker particle correlations in nucleus-nucleus colli-
sions than those in proton-proton collisions, as long as no space-time
structures and no coherent mechanism are considered. Observed pair data
does not seem to support an idea of statistical obscuration of correlations. "
Recent development in QCD lattice calculations I and experimental
studies of high multiplicity/high energy density phenomenaZ encourage
searches for signatures of Quark-Gluon-Plasma, a new state of matter.
Central or quasi-central collisions are most promising to realize the
required high density for QGPtransitions, while peripheral collisions are
least likely to contain possible QGPsignals. Inclusive s_udies of colli-
sions are generally governed by a large fluctuation of impact parameter
which only enhances3Peripheral phenomena, and are very insensitive to theinteraction models. Unlike these insensitive parameters (<N> and <N>/D),
an exclusive character of pair abundances may not be much obscured even in
https://ntrs.nasa.gov/search.jsp?R=19850027580 2020-03-20T17:21:17+00:00Z
169.
HEI.4-5
an inclusive analysis. We examine in this paper whether there are any
non-trivial abundances of narrow hadronic pairs in both individual and
inclusive nucleus,nucleus interactions•
2. Methods• The charged tracks emanating from the first collision ver-
tices are measured for individual events• The psuedo-rapidity (n) and
azimuth angle (@) of a track are given by spatial coordinatesmeasured at
many emulsion layers downstream the vertex. Secondary interaction
tracks and early y. e+e" conversions,are removed from the track data as
much as possible. (Fine triangulation of tracks allows this elimination
of contaminationswhose origins are located at more than 50 - 150 _m down-
stream the first vertex, depending on n and vertex depth.) Obvious nuclear
fragments (including spectator protons) are also excluded from the data.
All charged tracks thus defined are used for obtaining the following
three different measures of pair correlation.
No. of pairs (An _ _, A_ _ _) (1)
[MeasureI] P(_ _, 8) z No. of all charged tracks (Nch)
[Measure II] W(_z _ An < _2) _ /_2 dN d(An) (2)
- _ d(An)d(A_)
dN R2 coshIan)2- cos(A@)2, or[Measure III] S(all) _ d--_z-, with _ { (An) + (a@) (3)
Measure I is useful in the analysis of individual events, and Measure II
enhances narrow pairs. Measure III convolves the correlation of the ra-
pidity with that of azimuth angle over the entire available n - @ space.
g
Byes: N_*: Muaber of PaLrs Eve._ Hame; Humbar of Pa_ro Event H_............__e_Number of
Zp(TeVln). ZT Zp(ToVln) + ZT ZP(TeV/n) + _ Pa£rm
* Nob + (Hf) J • 5° 10° 150 . H©h+ (Hf) J • 50 t0° 150 . Heh + (Hf) I • 50 10° 15°
3. Results An exam- A-,, _T, _ _ ,-_,_ _TA,_ _, _O _-., DATA._ ,_• I_'( 3 ) + C DG.I 13 Y3-T-._) + C BO.I 3 5 8 9"-C I ) + C O.I 24 39 55
ple, event (G12), shows ..o_. BG,II8 12 16 .72:h. EG.II5 7 9 .150eh+(S_ ..II27 4, _
A-S2: • _TA _ DATA_ 15 _-20: _A }7 60ndant pai i its _522s -{_6)o_.CDa.zl_z_abu rs n .,,o_ ,o.., , , oL.c '_'B_.II 7 11' _ - • .II 272O
forward rapidity regio '-_: _'A'_ _' " D-,_ _ATA5_. . o._: _TA_ _ ,_* BO.II 8 13 18 ,1_9_110):h _.1I 25 37 50 6(3_) ch. Bg.lI_'I 1 & 6
Fis la and ib show its ,--: DATA,_. . ,-_,o_ DATA_0,'5,5°o.,_: _T,,_,: ,_
linear target diagram and ""°_ ,o,,,,_ ,, ._oo.. ,o,,_ ,, ,, ... ,,
_TA 3_ A' 65 G-l: DATA l_ 27 38 G-Z0_: DATA 13 2; 32
eudo rapidity distri _,_)+_ ,_., _ _o _-(_).o ,o., ,_ ,, .-_,)+o _., ,o . .ps - - + 117 oh, EG.IZ 12 le 2_ * 101 eh. S°.II 13 19 26 +176(100)+(2a*) .ZI 15 22 31
h£&h*necE_ TeV/n) _d _d_ _lC_pltc£t_ _70 Hch • 2_0).bution, respectively. ._., "=_"o"............... _ _"
• hcksro_d ei_*_ (_.1 _d _,lI) mre w£_houc _d vLth oh* HBTI_ecc. (P_el_/nsry)
It is interesting to ,,.o...,o..o_..,..... ,......o, ....,.... _,_.,,..,
note that these pairs in this event seem to form a "ring'_ structure in ra-
pidities as well as a "jet" structure in the azimuth plane. (This event also
contains two "direct e+e" pairs" [me+e- being _.I00 and 300 MeV] in the
same forward rapidity region.) Measure I is used for individual event analy-
_,
. __ F___. lb
_lrs
BIAS_ _Irs *
(e*) _lrs *
...... _ _.___-_.,' • o,
"I 2 4 6 _ 7 _ _ 10H - 745_m
170
HE 1.4-5
ses at high energy (2 1 TeV/amu). Table. I gives P(< 0.2, 15O)x Nch for
15 events (1532 tracks) with backgr_d expected from random coincidence
of two particles(BG I) and that includes the HBT effect(BG II) The data
show significant excess above background levels for several events. (Four
events(B22, D2, F250, G20) show anomalous pair excess, while the other II
events give P(s 0.2, 15o , - BG II) = 6.9 %.
For low energy events the statistics (106 events for N_ _ 2; 3,695
tracks from 20 - 60 Ge_amu Fe + C, AgBr and Pb reactions) is sufficient
that both Measure II and III are taken for all samples. W(O _ An _ 0.I)
and W(O.1 _ An _ 0.2) distributions as a function of A@are given in Figs.
2a and 2b, respectively. Since W = dN/d(A@) is constant for a random back-
ground (shown in Fig. 2 by dotted lines), a signal of pair correlation is
apparent in the region
A@< 30o. ':In \jj
The same signals ,, : o._ 7o ,, : o.2
can be seen in the in- In n
clusive Measure III as _5o
well, though the S/N  ULnnnn- -{L nnlnn
ratio is obviously re- _L...._._..x_l,_J[_]___ d,
duced in dN/dR2 as a 3_ -- - ____ii_!ii},_.!!!result of inclusion of 150i¢___i_8__30_16_ 60
large population at A_
> 0.2. In Figs. 3a
and 3b, the purely ,........... ,....., .....,..... ,, .....,.....,.....,.....,.....j.....
6_ _ 12_ 1B@ o° 6_ a_ 12@ 18_random (n, _) back- _,G_, n_
ground is a flat distri-
bution (dotted lines), while a random distribution in @ only is evaluated
by a Monte Carlo method; using real n and randomly reassigned @values:
(dashed-dot lines). The remainder in Figs. 3a and 3b indicates that
there are non-trivial rapidity- and azimuth-correlations.
12- R2 _ AI 12- R2 _ A2
Data : cosh(An) -cos(A@) Data : (An) 2 + (A@)2
10 _ I0
__- dN
8": 8
6 : i l i , I I s . J _ i . . , i , i l i •
0 0.1 0.2 AI0.3 0.4 o.s 0 0.1 0.2 A20.3 o.h o.s
Fig. 3 a. dN/dR2 (R2 _ A1) Fig. 3b. dN/dR2 (R2 _ A2)
4. Discussions. Results presented above indicate a significant non-
trivial correlation of short range nature. To evaluate the magnitude of
signals we introduce a trial source of a hypothetical parent meson (X)
decaying in two pions (or Kaons) with very small Q-values (_ 100 MeV).
Non-random signals (P(Data)-P(BG I)) are estimated for elev#n high ener-
gy events (An _ 0.2, A@_ 15°), and low energy events for (An _ 0.I, A@
30o ) and (An s 0.2, _ _ 30o ) in terms of x/x _ 10.2 %, 8.2 % and 13.9 %,
respectively.
Let's discuss non-random backgrounds. First, Dalitz pairs in o +
ye+e - decay(branching ratio: 1.2 %) can be contained in above x/_ ratios.
171 HE 1.4-5
However, this accounts for only 1.2 x (_o/(_+ + _.)) = 0.6 %. Next, some
early conversions of y-rays can contaminate the data. As mentioned earlier,
our track identificationis not efficient for eliminating them within 150_m
from the vertex, we expect this contamination upto (150/344,000)= 0.04 %.
Even when we fully include proposed internal bremsstrahlungsources4 in
addition to _o .XX photons, they are not able to contribute more than 0.2
%. Thus, electron backgrounds from both internal and external conversions
do not account for more than 0.8 %. Other experimental errors possibly in-
herent in measurements are not critically examined at present.
The Hambury-Brown and Twiss effect,5 the fourth order quantum inter-
ference of Bose-Einstein particles, may possibly explain some of the obser-
vation, since this effect is of very short range nature for nucleus-nucleus
collisions: d_d_2d2u d_ d_ __2(_2-C E _0 / ( 1 2 ) = 1 + exp( - ). (4)
The hadron source in nucleus-nucleus collisions is considered to be ex-
panded to the size larger than the colliding nuclear volume, and its known6
radius 3 - 4 fm gives the effective momentum difference lq] < I00 MeV.
This is sufficiently small that _+_+ and _-_- form narrow pairs (mostly in
Aq < O.l region). The HBT estimate by eq.(4) and flat q distributions gives
P(HBT) = 2 x 0.5 x n/2C2 x aB/(2_{In s - yo})/(n/2) = 3 ~ 7 % at A@ < 30o.
(Note that this approximation depends on the event multiplicity and effective
rapidity range.) An enhanced measure W(O _ Aq % .l) would contain 0.5 x
fWb.g.d(A@) HBT pairs, where the range of the integral must cover A@ upto .5x
tan-_(lO0 MeV/<PT>) = 30o. It is noticed that the observed pair correlation
increased for Aq _ 0.2 (see Fig. 2b), while the HBT particles should not.
This, with the analysis in _, favors consideration of some hypothetical
parent particles (X).
Freier and Waddington7 showed similar pair abundances in 2900 tracks
obtained from much lower energy (_ 7.5 GeV/amu) collisions. We estimated
P(O 5 An _ .I) = 255/2900 = 8.8 %. For this result similar to our low
energy data (20 - 60 GeV/amu), an HBT estimate explains about 67 pairs,
which, with 20 Dalitz pairs, accounts for 35 % of all extracted abundances.
In conclusion, about 30 ~ 50 % of non-random signals can be attributed
to the HBT effect from a large nuclear volume (3 ~ 4 fm). Nevertheless,
there seem to remain some "unexplained signals" of narrow hadronic pairs.
The net signals for "X" (not attributable to known backgrounds) in the
present analysis become X/_ . ~ 7 %; 3.5 % and 6.3 %, for three groups
defined previously. A particular group of events at high energies that had
the enormous X/_ _atio(~30 %) are by no means reconciled in the present anal-
ysis. All these results might require further examinations of systematic
errors and background estimates before seriously considering (X) sources.
Further experimental and theoretical analyses of hadronic pairs seem to
be interesting, not only because unexplained abundances still exist, but
- also because some exclusive characteristics of QGP, and particularly Chiral
phase transitions, can be considered in the narrow pair phenomena. °
References: I._ J.Polonyi et al., Phys. Rev. Lett. 53, 644(1984). 2. T.H.
Burnett et aI.(JACEE), Phys Rev. Lett. 50, 2062C1983). 3. T.H.Burnett et
aI.(JACEE), AIP 123, 723(1984). 4. J.D.Bj--orkena d L.McLerran, Phys. Rev.
D31, 63(1985). 5. R.Hambury Brown and R.O.Twiss, Nature, 17___77,2 (1956).-
_. W.A.Zajc et al., LBL-12652, 350(1981); D. Beavis et al., Phys. Rev. C27,
910(1983). 7. P.S.Freier and C.J.Waddington,AIP 49, 87(1979). 8. Y.
Takahashi and P.B.Eby, this conference HE 1.3-13.


Observation of direct hadronic pairs in nucleus-nucleus collisions in JACEE emulsion chambers

Observation of direct hadronic pairs in nucleus-nucleus collisions in JACEE emulsion chambers

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